The association of autoantibodies having an apparent specificity for anionic phospholipids with venous and arterial thrombosis, recurrent fetal loss, and thrombocytopenia is recognized as the antiphospholipid antibody (aPL) syndrome. It is hypothesized that autoantibodies associated with the aPL syndrome directly contribute to a thrombotic diathesis by interfering with hemostatic reactions that occur on the surface of blood cells and vascular endothelium. Important new insights into the pathophysiology of the aPL syndrome have been provided by recent evidence these autoantibodies do not recognize anionic phospholipids alone, but are directed against a number of phospholipid-binding plasma proteins and proteins expressed on the surface of vascular endothelial cells. Certain autoantidies target proteins that are components of, or may interact with, the protein C pathway, a physiologically important natural anticoagulant mechanism. The proposed studies will test the hypotheses 1) that hypercoagulability in a large proportion of patients with the aPL syndrome is due to autoantibody-mediated inhibition of the protein C pathway by specific autoantibodies and 2) that autoantibodies capable of inhibiting the protein C pathway are strongly associated with the clinical features characteristic of inherited defects of the pathway, i.e., venous thrombosis and recurrent fetal loss. The goals of the current proposal are to determine which autoantibodies associated with the aPL syndrome inhibit the protein C pathway, the clinical manifestations associated with such autoantibodies, the point in the protein C pathway at which the autoantibodies act, autoantibody properties that influence their inhibitory activity, and whether antibody-mediated inhibition of the protein C pathway is associated with in vivo thrombogenicity. The specific aims of the proposal are 1) the identification, characterization, and purification of autoantibodies directed against protein C protein S, thrombin, thrombomodulin, factor V, and beta2GPI, and correlation of these autoantibodies with clinical manifestations of the a aPL syndrome; 2) the production and characterization of human monoclonal antibodies with these specificities from peripheral B cells of patients with the aPL syndrome utilizing a CD40 system, 3) determination of the effects of the antibodies characterized in the first two aims on reactions of the protein C pathway, and 4) evaluation of antibody thrombogenicity in an in vivo animal model of venous thrombosis.